Modern radar and telecommunications systems require high frequency signal sources and signal processing systems with stringent performance requirements and extremely good spectral purity. Thus, there is a need for signal processing systems and signal sources with ever increasing spectral purity, stability and power-handling requirements.
Resonators, by their nature, provide discrimination of wanted signals from unwanted signals. The purity and stability of the signals produced is directly linked to the resonator used as the frequency determining device and is dependant upon its Q-factor, power handling ability and its immunity to vibrational and temperature related effects.
It is known that a piece of dielectric material has self-resonant modes in the electromagnetic spectrum that are determined by its dielectric constant and physical dimensions. The spectral properties of a given mode in a piece of dielectric material are determined by the intrinsic properties of the dielectric material, its geometric shape, the radiation pattern of the mode and properties and dimensions of the materials surrounding or near the dielectric material.
Prior art resonators have traditionally relied on metallic cavities containing no dielectric material, or on metallic cavities containing a dielectric material which were limited in Q-factor by the properties and dimensions of the metallic cavities. These prior art resonators were commonly operated at cryogenic temperatures in order to obtain a better Q-factor. However, to maintain cryogenic temperatures requires equipment which is cumbersome and difficult to incorporate into a portable or compact apparatus.
U.S. Pat. No. 5,712,605 to Flory and Taber describes a resonator structure that seeks to address these problems. The resonator described in U.S. Pat. No. 5,712,605 is a complex stack of hollow cylinders and flat discs formed of dielectric material. The cylinders and discs are enclosed within a metal cavity, with the hollow cylinders and discs forming a series of axially aligned cavities. The length of the cylinders and the diameter of the discs determine the operating mode of the resonator. The resonator is described as offering a high Q-factor.
Although the resonator described in U.S. Pat. No. 5,712,605 offers a high Q-factor, there are several disadvantages associated with the resonator structure. These include the difficulty of manufacture and its sensitivity to vibration. The device is difficult to manufacture because the hollow cylinders must be perfectly coaxial or the operation of the resonator will be significantly impaired. Further, because the resonant cavities are defined by the dielectric discs and hollow cylinders, any vibration or movement induced in one or more of the dielectric hollow cylinders or discs will result in a corresponding change in the shape of the resonant cavity, with a resulting change in the resonant frequency. This is referred to as mode breaking and has limited the usefulness of this resonator structure.
C. J. Maggiore et al describe a further resonator structure in their paper “Low-loss microwave cavity using layered-dielectric materials”, Appl. Phys. Lett. Vol 64 No 11, p1451. This resonator comprises a hollow, cylindrical copper cavity with one to four circular dielectric plates placed in parallel and axially spaced within the cavity. The Q-factor of the resonator was observed by Maggiore et al to increase as more dielectric plates were used.
Maggiore et al acknowledge at p1453 that although the Q-factor of the resonator at room temperature is high enough to have application to frequency stabilised oscillators, it will be necessary to thermally stabilise the cavity. This is because the dielectric plates are held in the cavity by means of circumferential grooves cut in the cavity wall. Copper has a thermal expansion coefficient of 16.8×10−6; thus a 1° Celsius temperature change will produce a 3.5 MHz change in operating frequency of a 19 GHz resonator, which is due to the change in spacing between the dielectric plates resulting from the expansion/contraction of the copper cavity.